Genes associated with specific autosomal recessive syndromes may predispose the heterozygous carrier to cancer and leukemia. Because heterozygotes for these genes are relatively common in the general population, some genes identified in this way are likely to be among the important human neoplasia-predisposing genes. Fanconi anemia (FA) and ataxia-telangiectasia are two autosomal recessive syndromes associated with cancer and leukemia in heterozygotes and homozygotes. A systematic procedure has been developed for analyzing the defect determined by the FA gene, based on the observation that several abnormalities of FA fibroblast cultures are corrected by co-cultivation with normal cells. This procedure may be used to analyze the basic metabolic abnormality of several important neoplasia-predisposing genes.